Validation of the definition of jkeywordp with respect to the ABNF grammar of Java.
The predicate jkeywordp defines (non-restricted) keywords `directly', i.e. without reference to the grammar. Here we introduce an alternative predicate based on the grammar, and we show it equivalent to jkeywordp.
We only perform this validation for non-restricted keywords, and not for restricted keywords (see restricted-jkeywordp, because the latter do not have a grammar rule.